GEOSTRATA, 2015, May/June, 16-18.
Port and harbor projects challenge geotechnical engineers with a combination of soft ground, large loads, exposure to natural hazards, and constructability issues in the marine environment. Successful designs achieve an optimal balance of attention to constructability, efficient operations and life cycle management, and sound environmental practices. This project-specific balance requires thoughtful, multi-disciplinary engineering where teamwork and ingenuity are highly valued. With the goal of promoting creative exchange between members of the Geo-Institute and the Coasts, Oceans, Ports, and Rivers Institute (COPRI), this commentary addresses three issues involving geotechnical applications at the interface of the maritime and surface transportation systems.
Waterfront structures such as piers, wharves, and bulkheads can tolerate deformations of foundations and earth retention systems within prescribed limits, often much larger than commonly applied for onshore projects. Deformations arise from long-term consolidation and creep of foundation soils under static loads combined with soil response to transient loads due to operations and natural hazards (e.g., berthing and mooring, wave, flood, seismic).
The port engineering community has adopted performance-based design methods to reflect this deformation tolerance. The recently published ASCE/ COPRI 61-14 Seismic Standards for Piers and Wharves addresses multiple levels of performance that are linked to three seismic hazard levels, modified by the importance of the waterfront structure. The target levels of performance include: minimal damage, controlled and repairable damage, and life-safety protection. From the geotechnical perspective, this design approach requires evaluating the magnitude and, very importantly, the pattern of the ground deformations for the three performance levels.
The demands for geotechnical input in support of performance-based design (i.e., “deformation-based” evaluation) can be significantly more involved than for General Limit Equilibrium (GLE) analyses. For port development, this requires determining if the potential exists for hazards associated with excessive soil deformation, computing the magnitude and pattern of the deformations, and evaluating the potential impact of these deformations on the performance of foundations and earth retention systems. Two items are necessary aspects of this assessment: thorough site characterization of both onshore and near-shore conditions, and the integration of in-situ and laboratory testing sufficient to support dynamic modeling and deformation computations. The application of instrumentation and advanced imaging of waterfront structures and foundations to confirm post-construction performance would greatly contribute to future improvements to the design methods and should be considered a third important part of the assessment.
Onshore/Offshore Site Characterization
Geotechnical, hydrodynamic, and marine conditions are commonly quite variable throughout the port system. The site characterization plan should be developed with this in mind and integrate the use of geotechnical and geophysicaltools available for both onshore and marine environments to provide necessary geologic and geotechnical data. These data can include surface topography, bathymetry, and vertical and horizontal variability of soil stratigraphy highlighting weak, compressible deposits that may negatively impact the performance of the waterfront structures. Time-dependent changes in site conditions should be anticipated (e.g., consolidation settlement, scour, sea-level rise). While these may seem generic and well-worn refrains, many geoprofessionals might be surprised and impressed by the array of geophysical methods being used in the marine environment to supplement more routine methods of geotechnical site investigation. Increased use of these offshore technologies would improve the stratigraphic and properties characterization upon which performance-based design relies so heavily.
Integration of In-Situ and Laboratory Testing
The general requirements for geotechnical investigations of port waterfront structures have been highlighted as follows:
The geotechnical properties shall account for possible temperature-, time-, and stress dependent changes (e.g., changes due to consolidation, scour). The dynamic properties will account for the intensity (cyclic stress/strain levels), frequency, and duration of the loads. Dynamic loads transferred to foundations, anchors used for mooring systems, and earth retention systems due to wave action, seismic, wind, berthing and mooring, and other operational live loads shall be considered
G-I members can appreciate the scope of the laboratory testing required to satisfy these broad goals. The development of the “optimal” testing program, particularly when dealing with weak marine soils, is part of the creative process that melds a command of soil behavior with an understanding of the geotechnical input parameters required for project specific analyses.
Instrumentation to Support Performance Assessment and Model Calibration
Evaluating the margin of safety against bearing capacity and slope stability failure using GLE methods is a necessary first step in the design process; however, safety margins are often low in the weak soils that are prevalent in the marine environment. The factors of safety obtained from GLE methods of analysis are only indirectly related to ground and structural deformations that govern overall performance. This has led to the utilization of sophisticated 2-D and 3-D numerical geomechanical modeling for estimating the patterns of deformations of foundation soils, waterfront slopes, and structures. Substantial advances have recently been made in modeling pile-supported wharves and bulkheads; however, the lack of well-documented, instrumented field case histories has precluded thorough calibration of analysis methods for simulating dynamic soil-foundation structure interaction of these structures. The lack of model calibration can lead to a poor understanding of the uncertainty involved in seismic analyses and an over-confidence in the analysis results.
The use of instrumentation on and adjacent to port waterfront structures has generally been limited due to the extreme environment, construction-induced loads, and limited longevity of dedicated instrumentation. These impediments notwithstanding, it is believed that the enhanced use of instrumentation and imaging (ranging from MEMS to InSAR), and geophysics for monitoring would greatly benefit performance assessment.
Increasing the reliability of geomechanical modeling in support of performance-based design at ports will require the integrated application of site characterization technologies developed for use in onshore and offshore environments, in-situ and laboratory testing programs developed with the modeling requirements firmly in mind, and an improved understanding of soil-structure interaction during operations and extreme loads. The creative, multi-disciplinary exchange encouraged in this commentary will hopefully greatly accelerate innovation in geotechnical engineering at ports.